Artificial Underground Water Heat Accumulator

ABSTRACT

Disclosed is a device of a water heat accumulator or storage medium that is composed of water and at least one solid construction material which has a structure with hollow spaces and is specifically designed to absorb the entire amount of water and autonomously retain as much thereof as possible, has a statically load-bearing structure and can thus accommodate a superstructure, and can have water-permeable and/or hygroscopic properties. All additional equipment required for operating and utilizing the heat accumulator is assigned to said water heat accumulator or storage medium. The inventive device is characterized in that the construction material is a foam product or a construction material which is composed of several identical and/or different solid construction materials that are fixedly interconnected.

TECHNICAL FIELD

The present invention generally relates to a water heat accumulator, and more particularly, to a water heat accumulator comprising a water-absorbing and load-bearing material.

BACKGROUND OF THE INVENTION

Artificial water accumulators are water collection units that may be built aboveground or underground. Depending on need and specific purpose they may take the form of small or large containers or collecting tanks. Because water is a liquid medium it must be collected in a sealed container. Depending upon the selected structural form, such water accumulators must be constructed in a way that is statically load-bearing, and are accordingly very expensive to manufacture, either in a factory or on-site. Water accumulators are generally built either as containers made of steel, of fiberglass-reinforced composite material, or as a reinforced concrete structure.

Water containers have been used as heat accumulators, because water is known to have very good heat transportation and storage properties. Water is available in large quantities on a very affordable basis, at least in Central Europe. Heat storage covers a very broad and multi-faceted field. This ranges from smaller heat accumulators for daily use, as is known from building engineering, so-called short-term heat accumulators, up to large and very large long-term heat accumulators for low-temperature heat.

Heat accumulators are used to balance a supply of heat with variable demand over time. Heat accumulators enable storing and efficiently utilizing (exhaust) heat that is generated discontinuously and in different quantities and temperatures, e.g. from industrial production, building and district heating technology, or even from wastewater and ground heat. Heat may be stored either over the short term, as intermediate storage, or seasonally, i.e. long term. Solar-thermal short-term and seasonal long-term storage is being researched and tested in pilot projects. Solar-thermal heat storage is becoming increasingly economical due to the increasing scarcity and thus increased price of primary fossil fuel reserves. A good overview of the general state of possible technologies is provided by the publication entitled “Langzeit-Wärmespeicher und solare Nahwärme (Long-Term Heat Accumulators and Solar-Based Local District Heating)” published by the BINE Information Service of Fachinformationszentrum Karlsruhe (FIZ Karlsruhe), located at Mechenstraβe 57, 53129 Bonn.

According to the physical principle, heat accumulators are distinguished between tangible/sensible, latent, and chemical heat storage. According to the storage medium they are differentiated into water, stone or gravel-water accumulators. Among accumulators for tangible heat, the dominant types are those that use exclusively water or a mixture of water and stone as a storage medium. There are also geothermal vertical loop storage systems, hybrid accumulators, and aquifer accumulators, which utilize the natural geology and/or the soil as a heat accumulator and heat source, and make use of processes which doesn't require digging, e.g. drilling, to recover heat. Often artificial, water heat accumulators are also integrated into these systems as buffer containers in order to increase efficiency.

A distinct characteristic of heat accumulators is that their top and side walls must be thermally insulated. Thermal insulation on the bottom of the container may be omitted under certain circumstances, if the container is embedded in the ground. To minimize heat loss and the expense of thermal insulation, efforts must be made to achieve an optimal geometry of the storage volume, which is achieved by reducing the surface in proportion to the volume to the extent possible. In this case, the theoretically ideal form would be a sphere, a cube, or a cylinder with identical diameter and height.

There is also the desire to embed the storage medium in a heat accumulator with stacked layers of different temperatures. The cold water layers will then preferably position themselves at the very bottom, and the increasingly warmer water layers will position themselves above. This is possible because water has relatively poor heat conductivity, and thus it has been shown that there is very little temperature exchange between hot and cold layers in a preferably undisturbed storage medium. Thus the ideal storage geometry would be taller and narrower rather than flatter and broader, so as to minimize the contact surface between the different storage levels. Thus the ideal proportions for a narrow cylindrical accumulator would be a height/diameter ratio of 2:1, or even better 5:1. In addition, cylindrical containers have a higher static load-bearing capacity vis-à-vis internal pressure simply because of their rotation-symmetric shape. These more or less theoretical considerations are subject to practical limits, however, because in each case this geometry always requires very elaborate, i.e. statically load-bearing, containers made of steel, concrete, or fiberglass-reinforced composites.

Moreover, heat accumulators that use exclusively water as a storage medium require an elaborate cover supporting structure. The requirements for this cover increase as the size of the container diameter increases, and also if there will be vehicular traffic or structures built over the cover in order to maximize space utilization. This is because in the case of structurally compact local district heating infrastructures, where it makes the most sense to use such accumulators, every square meter of space is valuable, and must be used to the extent possible for building structures, or at least for landscaping and roadways. With regard to the design and dimensions of long-term heat accumulators, therefore, the search continues for a compromise that would make this technology technically as well as economically feasible. The currently preferred form is a reinforced concrete container that is partially embedded in the ground, banked with soil, and built over, with a base and a load-bearing cover in the form of a frustum. The height/diameter ratio is approximately 1:2.

These accumulators work at temperatures of up to approx. 100° C., and thus have very high quality demands on the construction materials used in order to achieve the necessary fatigue strength. This applies to the container structure itself as well as to the thermal insulation and the generally required additional waterproofing. Many composites are not suitable because of the high temperatures, and thus only a comparatively very expensive corrosion-resistant internal lining of stainless steel or fiberglass-reinforced composite can be used, and the reinforced concrete structure must be made of a special concrete mixture.

Because the capacity of long-term heat accumulators is used only once or twice per year, and the aforementioned effort has ultimately not yet led to an economically acceptable result, increasingly large storage volumes are needed to reduce the volume-based investment costs and improve the cost-benefit ratio.

An interesting variant for saving construction costs compared to container accumulators is the so-called pit water heat accumulator. In this structure, a pit or trough is excavated, lined with waterproofing material, thermally insulated, and filled with water. The cost advantage of this design is that it does not require a load-bearing steel-reinforced concrete design for the collecting tank in the previously necessary dimensions. Nevertheless, these pits are increasingly unable to fulfill the aforementioned requirements for an optimal geometry. Accordingly, the accumulator is built as a relatively shallow structure and requires a large area, causing heat losses to be high, unless lower water temperatures are used. The latter option, however, once again results in a lower specific output per cubic meter of storage volume, and requires an even larger accumulator.

Another disadvantage is that the load-bearing cover thereby becomes even more elaborate, although there are attempts to design the cover as a floating structure. Its structural execution has nonetheless been difficult thus far, particularly due to the volume change of the water and thus the water level. Another variant of the water heat accumulator for long-term low temperature heat storage in the form of the gravel-water or soil-water heat accumulator can provide assistance in this case. In this variant, a mixture of gravel and water is used as the storage medium and is employed primarily in connection with a pit water heat accumulator. A load-bearing roof structure is no longer required for this design because of the statically load-bearing gravel portion, and thus building over the structure is not a problem. The use of gravel also has disadvantages, however, because this construction material takes up approx. 60-70% of the storage volume, and due to the lower heat storage capability of this material compared to pure water heat accumulators, it requires a total storage volume that is approx. 50% larger in order to be able to store a comparatively equal quantity of heat. Gravel is also approximately 10 times more expensive than water, if it cannot be found and appropriately cleaned on site during excavation, and must instead be delivered. One could also use the simple refill from the excavation, but would then require approximately twice as much storage volume for the same heat output.

A more detailed design of a corresponding pit water heat accumulator variant that is filled with water-saturated excavated soil or water-saturated fill containing loose particles or granulate that can also absorb water is disclosed by German patent DE 199 29 692. Containing the refilled soil and/or the individual particles or granules in a pit that is lined with a watertight sealing layer or film is necessary to ensure that the refilled soil or particles support each other reciprocally and against the walls of the pit, and thus can form a supporting structure that can bear horizontal and vertical loads of a contained aggregate material against the foundation. The same principle applies to all soil-water or gravel-water heat accumulators and/or pit water heat accumulators of the prior art.

Another disadvantage of known water heat accumulators is that leakage and corrosion can cause the accumulator to lose water and heat. Moreover, the leaking water would wash out the area underneath the heat accumulator or its foundation, and thus the statics of the structure would be endangered over the short or long term, in other words a scenario that could end in the failure of the entire project.

SUMMARY OF THE INVENTION

In an exemplary embodiment of the present invention a water heat accumulator is disclosed that retains water in at least one storage medium. The storage medium is made of a solid construction material which comprises cavities specifically designed to absorb and retain a large amount of water. The storage medium may form a statically load-bearing structure, which may accommodate a superstructure. The storage medium material may have water-permeable or hygroscopic properties or both. Equipment required for operating and utilizing the heat accumulator may be mounted to the storage medium. The storage medium does not necessarily require a statically stable surrounding structure for its manufacture and operation. Due to its simplicity and cost-effective manufacturing and installation, the new construction material can be used to cost effectively build very small heat accumulators with sufficient output.

In order to install water heat accumulators in existing structures, the storage medium must have sufficient static strength to support superstructures, e.g. buildings, without large additional expense. This presents a technological challenge, because once superstructures have been built over a water heat accumulator, it is no longer accessible. This requires the use of materials which are long-term stable, rot-resistant, maintenance-free and environmentally safe. The accumulator and/or the water contained in it must not pose any risk to the foundation below large superstructures. Use of a construction material that is a foam product or a construction material that consists of several identical and/or different solid construction materials that are permanently bonded with one another may satisfy those requirements.

Cement-type foam products, also known as foamed concretes, are commonly used in construction, where they primarily serve to fill hollow cavities or as a building material. Foamed concretes are light, low in material content, and provide good thermal insulation. Foamed concretes serve their primary purpose only when they are largely free of moisture and wetness. For this reason, foamed concrete structures must first dry before they can be used for further construction or for their intended purpose. Moreover, such construction materials are conditioned so that after they have dried they absorb as little moisture as possible, even in the event of wetness from outside. Foamed concrete structures may also be protected against external moisture through exterior covers or insulation layers.

Contrary to its traditional use, the construction material in the disclosed embodiment absorbs water, primarily for heat storage. The construction material may be a foam product that absorbs water instead of air in its solid foam bubbles. The disclosed construction material offers an advantageous ratio of stored water volume per construction material volume. Similar to other cement products, this construction material can be easily manufactured, marketed, transported, and quickly processed. The solid structure of the construction material can then be filled and/or saturated with water, and can accommodate a superstructure. Such a heat accumulator can be installed very cost-effectively with the largest possible storage capacity, even in small individual units under new buildings. As known from other applications, the cement and/or concrete construction material itself has a long service life, is maintenance-free, and is environmentally safe. Nevertheless, the heat accumulator must then be operated as a quiescent water accumulator with an indirect heat input and output device, e.g. by means of built-in collector pipes.

In a further embodiment the construction material may consists of several identical or different solid construction materials that are permanently bonded with one another. A binding agent may be used that is cement-like, and consists of several solid construction materials. In addition to corresponding binding agents, various solid construction materials are commercially available from other known applications, e.g. including all suitable forms of particles, granules, other aggregates and/or fibers, and can be combined in a suitable matter for this purpose.

In another exemplary embodiment the construction material may be pumice stone. Pumice stone is a porous, glassy volcanic rock, the specific weight of which is lower (by approximately two-thirds) than water. Its water storage capacity is correspondingly high. It is may be used in connection with cement-like binding agents for the manufacture of structural stones, also as hollow pumice blocks or lightweight concrete construction blocks. A storage medium consisting of pumice granulate and a cement-like binding agent is statically stable even without an exterior supporting structure and has a good water absorption capacity because of its porous structure. The use of this construction material provides effective small heat accumulator units.

In yet another exemplary embodiment the construction material for the construction of a heat accumulator may be liquid at the time of processing and become solid after processing. This can be a construction material in the form of a binding agent, or a binding agent with other solid construction materials. The binding agent may consists of several construction materials. In this case, the binding agent is liquid and can be processed alone, e.g. as a foam product, or if necessary mixed with other solid construction materials, e.g. with pumice granulate, as a viscous concrete mass. This liquid construction material can be easily filled into collection containers, formworks, or molds. If the project involves spaces created by formworks, then the formwork is usually removed after the construction material hardens. The construction material can also be processed without additional formwork, however, for example if it is directly poured into a corresponding pit. The storage medium may also protrude partially or completely from the soil. In these cases, at least a partial formwork and in this regard certainly also an additional framing, e.g. with a concrete wall, is necessary, wherein the concrete wall can be used simultaneously as the remaining formwork for purposes of efficiency. At a minimum, however, the storage medium will have a virtually flush abutment with the superstructure, regardless of whether a structure being built has a basement.

Liquid construction material may be poured into a bare pit, where it penetrates into the peripheral zone of the adjacent soil, which becomes saturated with the corresponding binding agent. Once the binding agent has hardened, this peripheral zone becomes a statically stable concrete wall that is largely sealed against the storage medium and transitions almost seamlessly into the construction material of the storage medium. The concrete wall may also provide thermal insulation. The heat accumulator thereby automatically obtains an additional concrete shell without additional effort. In precisely the same way, imperceptible leaks in the concrete basin or in troughs lined with waterproofing material can be automatically sealed after being filled in with this construction material. A formwork will also always be helpful if one wishes to implement optimal heat accumulator geometries, in contrast to conventional soil basin design.

Depending upon requirements, e.g. if the depth of the heat accumulator means that it is located in soil layers that carry groundwater, it will be advisable to equip the storage medium with a complete or partial encasement to optimize its performance. In addition to a cover on the top, which must provide thermal insulation and waterproofing, depending upon the geological and/or hydro-geological conditions, the same procedure applies to the side walls and the bottom, although here the thermal insulation can possibly be omitted because the cold temperatures will preferably be stored in the lower area of the heat accumulator. The encasement can also serve as additional stabilization of the storage medium if necessary, to the extent that this is required under certain circumstances. Such an encasement can consist of a wide range of materials, among them composites and concrete. Waterproofing on the bottom may be omitted because the accumulator can largely hold the water itself solely due to the specific structure of the construction material and, as described above, it may seal itself during installation. Moreover, the storage medium may be sealed from above and from the sides with an encasement in the form of a cap in such a way that because of the vacuum formed underneath the cap, the accumulator loses almost no water downward.

However, the direct form-fitting contact of the storage medium with the adjacent soil can lead to successive water seepage, i.e. a minor loss of stored water cannot always be avoided. Water loss may be compensated by feeding additional water to the construction material structure in a controlled manner. Additional water may be inserted from above as needed between the actual storage medium and the encasement and/or cover. This is accomplished by means of conventional pipelines, valves, and measurement equipment, etc. used in pipeline construction. The degree of saturation and/or the water level in the accumulator can therefore be checked at any time and properly maintained.

The most geometrically suitable form of accumulator can be implemented either by on-site processing of the construction material in liquid form as ready-mixed or mixed-on-site concrete, or as a delivered, solid, prefabricated product. In another exemplary embodiment a complete heat accumulator together with its additional equipment can be prefabricated as a ready-to-install element in a factory. It may be delivered to the construction site, e.g. with a low-bed trailer. The size will of course depend upon transportation capabilities and will have an upper limit, although this is certainly an interesting variant for smaller buildings with correspondingly smaller accumulators. Because of its relatively low weight, the accumulator delivered in this way can therefore be placed into the prepared pit with a crane. After installation the accumulator is filled with water.

Similar cost saving may be achieved for larger accumulators by joining together several small and easy to manage prefabricated accumulators on-site.

In another exemplary embodiment a heat storage medium is disclosed which utilizes a solid foam product or multiple identical or different solid construction materials that are permanently bonded with one another. Thus, in contrast to the prior art, it is now possible to create in an advantageous manner large supporting structures and thus large storage volumes, even outside of the ground, without the need for a additional statically supporting surrounding structure. The disclosed heat storage medium may be filled with water after installation. The heat storage medium may act as a supporting structure and be prefabricated as a large block or cylinder in a mold. The heat storage medium may be a statically stable prefabricated construction unit, which is delivered to the construction site dry. On-site it is installed, and only then filled with the storage water.

Known water heat accumulators use a loose gravel-water mixture whereby the gravel occupies approx. 60-70% of the accumulator's volume and thus only 30-40% of the accumulator's volume is filled with water. In contrast the heat storage medium according to one aspect of the invention occupies the entire storage volume and absorbs water in internal cavities. The amount of water stored in the internal cavities is significantly larger than the amount of water stored in a gravel-water accumulator of same volume. These cavities may be pores, seams, cracks, capillaries, or a combination thereof. In principle, this therefore permits all forms from small to the smallest cavities that can absorb and retain water on their own. These cavities can be open or closed. In order to be able to saturate even closed cavities with water, the base material of the construction material can also have water-permeable and/or hygroscopic properties. Materials are known that can absorb and retain up to 80% of their own volume in water in the aforementioned matter. The water absorption capability ultimately depends upon the construction material used and its processing and/or its commercial supply. The material may be natural and/or artificial, and/or organic or inorganic.

Advantageously the new construction material may retain water that is absorbed in its cavities independently, i.e. without additional resources, up to a specific saturation point. This characteristic is inherent to the material, and based on the capillary effect acting on small to very small cavities and their connection. Consequentially the storage material may not require additional waterproofing to the outside, and eliminate the risk of water leaking from the heat accumulator to the outside. Preventing water leakage is important to avoid heat loss as well as flooding of soil surrounding the heat accumulator and potentially endangering the base or foundation of a superstructure build over the heat accumulator.

In some cases the additional external waterproofing that is required in prior art water heat accumulators can be omitted. This allows for smaller, more economical water heat accumulators. However, without external waterproofing a loss of water may occur, e.g. if the storage medium comes into contact with the stone in its trough. Water loss will occur in such small quantities, however, that only minimum heat quantities will be lost from the heat accumulator, and there will be no notable change in the natural soil structure. Lost water can easily be refilled from above by means of an appropriate water connection.

After the installation of the heat accumulator, the construction material will have a statically loaded-bearing solid structure that allows for buildings or other structures to be built on top of the water heat accumulator. Since the storage medium itself provides static stability no additional support, e.g. from a steel, fiberglass-reinforced composite, or reinforced concrete container or tank, is required. The construction material may therefore be placed directly into a bare pit, saturated with water, connected to the plumbing for operating the accumulator, and then have a building constructed on top of it. The foundation of a building may be placed directly on the heat storage medium. There may also be additional waterproofing and thermal insulation between the foundation and the storage medium. The building's weight is transferred through the construction material directly into the subsoil. This can also be done in larger accumulators and/or volumes, but is also an additional step toward smaller, more economical accumulators as they will be needed in future, particularly in existing urban infrastructures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vertical cross-section through a heat accumulator, which is located under the foundation slab of a building and as a storage medium has a solid construction material structure containing cavities and filled with water.

DETAILED DESCRIPTION

Referring to FIG. 1, a cross section of an exemplary water heat accumulator in which the principles of the present invention may be advantageously practiced is illustrated generally. As illustrated, the exemplary water heat accumulator 17 is embedded completely in the soil 10, thereby using the heat-storing properties of the soil 10 and the corresponding thermal insulating effect of ground heat. The upper part of water heat accumulator 17 has a cylindrical shape which extends into a lower part in shape of an inverted frustum. This geometry minimizes to the extent possible the skin surface of the heat accumulator 17 relative to its volume. Water heat accumulator 17 could also use a square or polygonal layout. In practice the shape of water heat accumulator 17 will also be determined by its construction costs. A rectangular layout is possible, but deviates from the optimal geometry.

Water heat accumulator 17 is closed with a top cover, comprising protective membrane 3, a PP film vapor barrier, and a correspondingly thick thermal insulation 5. A foundation slab 11, e.g. made of concrete, is located over the top cover of water heat accumulator 17. Foundation slab 11 may be placed on accompanying PE film 9, and a parameter insulation 8.

The upper section of the accumulator 17 is waterproofed and thermally insulated. Thermal insulation 5 is protected against moisture from the outside, e.g. caused by groundwater in the adjacent soil 10, by means of a drain mat 13. Drain mat 13 is covered on both sides with filter membranes 12. Any existing moisture will be absorbed by a drainage pipe 14, which is installed in a circumferential drainage trench filled with gravel 7, and preferably fed into a rainwater management system. Water collected in drainage pipe 14 may be used to fill heat accumulator 17.

Foundation slab 11 may be the foundation of a building which may or may not have a basement. Foundation slab 11 may also be the substructure for an outdoor area, street, or parking lot.

Until now, installing heat accumulators below buildings was not customary. This was because the cost-benefit ratio favored large or very large capacity water heat accumulators. Generally, however, there is no free space available in crowded urban infrastructures to accommodate such large building projects, and accommodation of a superstructure, even over a gravel-water heat accumulator, has thus far been considered problematic in terms of both space requirements and technical feasibility. Regardless of this fact, a water heat accumulator with a superstructure always has the advantage of very good thermal insulation from above and protection against surface water due to the building alone.

Water heat accumulator 17 further comprises pipes 2 for heat input and output and equipment for any required measurement and filling. The pipes 2 are fed through the accumulator's cover and through the foundation slab 11 into the building by the shortest route. The wall feed-through points normally necessary for this purpose, also known as wall sleeves, are thermally insulated and waterproofed in relation to the building and/or the heat accumulator 17.

Heat is transferred into (input) or withdrawn from (output) storage medium 16 through collector pipes and a heat transfer medium circulating therein. The double arrow shown in FIG. 1 illustrates such reciprocal action for the individual collector pipe arrays. The sketch symbolically shows three collector pipe arrays independently installed in the accumulator at different levels/heights, which can store the heat with different temperatures in the accumulator and then withdraw it from this area. Accordingly, the coldest temperature is at the bottom and the warmest temperature is at the top of water heat accumulator 17.

Measurement and filling device 6 is provided to initially fill water heat accumulator 17 with water and to refill it later if necessary. Measurement and filling device 6 measures the water level and the temperatures of the storage medium 16 and/or the water levels with different temperatures.

Storage medium 16 is made of a statically stable, i.e. solid, porous construction material that can be completely saturated with water. Storage medium 16 may not require external waterproofing or a statically stable surrounding structure. This is because the porous, solid structure of the construction material absorbs the subsequently added water and retains it by means that include the capillary effect. This is advantageous over known gravel-water heat accumulators which would collapse and spill water into the surrounding soil if there were no supporting structure and waterproofing.

Storage medium 16 may be made of cement foam concrete construction material that is produced especially for this purpose. It may also be composed of pumice stone granulate with a cement-like binding agent. The construction material is mixed with water prior to processing as ready-made concrete or mixed-on-site concrete, and poured into the prepared pit in a liquid state. The pit may have a simple formwork on the sides, which can be removed after the construction material is hardened. This allows for installing the waterproofing films 12, 13 and thermal insulation material 5 on the side walls of storage medium 16 after it has hardened.

Drain mat 13 with its associated filter membranes 12 and thermal insulation 5 may also be installed along the walls of the excavated pit before the construction material of storage medium 16 is poured. In this case a formwork is not required. Whether or not to use additional formwork depends on the depth of the pit, the extent to which a banking of the pit walls is possible or desirable, and safety considerations.

The lower area, i.e. the heat accumulator's base cup, may not need to be additionally waterproofed prior to the poring of the construction material, in which case the pit remains in its raw condition. Waterproofing and thermal insulation of the accumulator's base are optional. A concrete base before pouring the construction material forming the heat storage medium 16 is never required.

Simultaneous with the pouring of the construction material forming storage medium 16, the collector pipes which are connected to input and output pipes 2 and the pipe with the corresponding sensors of the measurement and filling device 6 will also be embedded in the concrete. Once the binding material and/or the concrete has hardened, the solid body of the storage medium 16 can be stripped and clad on the sides with the additional films, membranes, and insulating materials. The accumulator 17 is then filled in once again around its sides with soil.

Inherently, an additional concrete shell 15 is automatically formed seamlessly with the construction material in the soil around the accumulator without additional effort. This creates a certain additional external stabilization and waterproofing for the storage medium 16. This is based on the fact that when filling in the construction material, outwardly saturated binding agent penetrates into the adjacent stone structure of the pit, and after hardening forms a solid concrete mass together with the soil. Except for a narrow upper residual height, the entire accumulator is now completely filled out with this solid construction material in a formfitting manner. The residual height is then filled out with gravel 7 before the cover with the pipe pass-throughs is installed and sealed. The accumulator can then be filled with water. There are hardly any risks of construction defects with this technology because the storage medium 16 is self-supporting and to a large extent self-sealing and water retaining. It is also additionally supported by the cap-shaped sealing of the storage medium 16 in the area of the cover and the sides, which in the event of water loss creates a vacuum below the cap, and thus in the storage medium. This vacuum can also be measured with the measurement and filling device 6. In addition to a moisture sensor and/or water level indicator, the height of the vacuum can also be a parameter for the water level 1 in the accumulator. This should optimally adjust to the height of the gravel layer 7, and can be supplemented if necessary by means of the filling device 6. Filling the accumulator with water certainly takes longer than is the case with a gravel-water heat accumulator, because the construction material can absorb the water only slowly through the pores, capillaries, and its hygroscopic properties. This is not a problem, however, because there is no need to begin operations before the building that will use the heat is built above the accumulator.

While the present invention has been described with reference to exemplary embodiments, it will be readily apparent to those skilled in the art that the invention is not limited to the disclosed or illustrated embodiments but, on the contrary, is intended to cover numerous other modifications, substitutions, variations and broad equivalent arrangements that are included within the spirit and scope of the following claims. 

1-8. (canceled)
 9. A heat accumulator mounted in a pit in the ground, the heat accumulator comprising a heat storage medium, wherein the heat storage medium a) is solid, b) comprises cavities suitable to absorb and retain a liquid, c) is water-permeable or hygroscopic, and d) is load-bearing.
 10. A heat accumulator as in claim 9, wherein the cavities in the heat storage medium are filled with water.
 11. A heat accumulator as in claim 9, wherein the cavities in the storage medium are pores, seams, cracks, capillaries, or a combination thereof.
 12. A heat accumulator as in claim 9, wherein the heat storage medium is sufficiently load bearing to allow buildings and other structures to be built over the heat accumulator.
 13. A heat accumulator as in claim 9, wherein the heat storage medium is made of hardened foam.
 14. A heat accumulator as in claim 9, wherein about 80% of the volume of the heat storage medium consist of water.
 15. A heat accumulator as in claim 9, wherein the heat storage medium is made of foamed concrete.
 16. A heat accumulator as in claim 9, wherein the heat storage medium contains pumice.
 17. A heat accumulator as in claim 9, wherein the heat storage medium is made of pieces which are joined together solid.
 18. A heat accumulator as in claim 9, wherein the heat storage medium is formed by pouring a liquid construction material into the pit in the ground and letting it harden solid.
 19. A heat accumulator as in claim 18, wherein the heat storage medium comprises a hardened outer shell where the liquid construction material was in contact with the surrounding soil.
 20. A heat storage medium for use in a water heat accumulator, wherein the heat storage medium is a solid material containing a plurality of cavities which can absorb and retain a liquid.
 21. A heat storage medium as in claim 20, wherein the cavities in the storage medium are pores, seams, cracks, capillaries, or a combination thereof.
 22. A heat storage medium as in claim 20, having water-permeable or hygroscopic properties or both.
 23. A heat accumulator comprising two or more heat storage mediums as in claim
 20. 24. A heat accumulator as in claim 23, wherein the heat storage mediums are pre-fabricated elements which are joined together during installation of the heat accumulator.
 25. A heat storage medium as in claim 20, wherein the liquid is added to the heat storage medium after it has been installed. 